drm/i915: add HAS_FORCEWAKE flag to uncore

We have several cases where we don't have forcewake (older gens, GVT and
planned display-only uncore), so, instead of checking every time against
the various condition, save the info in a flag and use that.

Note that this patch also change the behavior for gen5 with vpgu
enabled, but this is not an issue since we don't support vgpu on gen5.

v2: split out from previous path, fix check for missing case (Paulo)
v3: Inline helper for clarity in testing flags

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325214940.23632-3-daniele.ceraolospurio@intel.com